In a four-cycle engine, a piston reciprocates in the interior of a cylinder block, and a crankshaft accommodated in the interior of an engine case converts the reciprocation of the piston into a rotation to be output to the outside. In the four-cycle engine, various mechanical losses increase with an increase in an engine speed of the engine. Among the mechanical losses, pumping loss is known. The term “pumping loss” refers to a resistance to the reciprocation of the piston, and is mainly caused by air, including a blow-by gas in the interior of the engine case.
Japanese Laid-Open Patent Application Publication No. Hei. 5-60000 discloses a four-cycle engine devised to reduce pumping losses. This engine includes a vacuum pump operative in association with a rotation of a crankshaft accommodated in an engine case and an air chamber that is connected to the engine case and is maintained at a negative pressure by the vacuum pump. The vacuum pump operates according to an engine speed of the crankshaft to suction air from the inner space of the engine case, thereby maintaining the inner space of the engine case at a negative pressure.
In the inner space of the engine, oil droplets or oil mist exist, for example, cooling oil injected to a back surface of the piston, and oil scattered from a surface of the crankshaft during rotation. Some four-cycle engines are equipped with a breather for separating the oil from the air to suppress suctioning of the air containing the oil droplets or the oil mist. In order to suitably separate the oil from the air, it is necessary to increase a capacity of the breather or to equip a centrifugal device to separate the oil from the air. This may undesirably increase a weight, a dimension, and a manufacturing cost of the engine.